1. Field of the Invention
The invention is concerned generally with correction of errors induced by noise in light produced by lasers and more specifically with correction of such errors in CPT frequency standards.
2. Description of Related Art: FIG. 1
Light produced by lasers has many applications. Among them are applications where how well the application works is directly related to the amount of noise in the output produced by the application. For present purposes, noise is defined as any irregularity which makes the application work less well than it would otherwise. In some cases it is possible to attack the noise problem directly by controlling the ultimate source of the noise. However, in other instances it is either impossible or impractical to sufficiently dampen the sources of noise generation. For example, one class of noise is produced by thermal effects, which can be reduced by cryogenic cooling, but doing that may render the resulting system impractical. In such cases, it is possible to correct for the noise and thereby mitigate its impact on the output produced by the instrument.
Noise can be either purely random, periodic or combinations of both. At any given moment of time, the magnitude of the noise component in a signal is either completely unknown or in most cases, predictable only in the statistical sense. Therefore for a given measurement it is impossible or difficult to differentiate the desired signal from the undesirable noise term. The measured signal contains a component that is noise combined with a component that is the desired waveform. To remove noise, we need a mechanism that can predict the noise that exists in a given measurement of the actual signal. If the noise can be predicted, the noise can be removed from the output signal. For example, in the case of simple additive noise, the noise prediction is subtracted from the corrupted signal. For any noise process that is mathematically reversible, noise prediction can be used to reduce or completely eliminate the noise from the output signal.
In applications involving laser light, two important kinds of noise are irregularities in the intensity of the laser light and irregularities in the frequency of the laser light. One application of laser light where the noise in the light is important is in coherent population trapping (CPT) atomic frequency standards, popularly known as “CPT atomic clocks”. What these devices do is use the light emitted by the laser to produce a signal with an extremely regular form. That signal can in turn be used to precisely measure intervals of time. FIG. 1 shows at 101 a high-level block diagram of a CPT frequency standard 101. A laser light source 103 of the proper frequency provides a beam of light which passes through a rubidium vapor cell 105. The frequency of the laser light is modulated such that the beam of light causes the rubidium atoms in the vapor to resonate. The resonations in turn affect how much of the laser light is absorbed by rubidium vapor cell 105 and thus how much of the laser light reaches photodetector 107. The output from photodetector 107 of course varies with the amount of light that reaches it, and the form of the output thus reflects the resonance of the rubidium atoms. The output is then provided to signal processor 109 for further processing. For more details concerning CPT atomic frequency standards, see U.S. Pat. No. 6,320,472, Jacques Vanier, Atomic Frequency Standard, issued Nov. 20, 2001, which is incorporated in the present application by reference.
As is apparent from the foregoing description, the accuracy of the output produced by photodetector 107 is affected by noise in the laser light. If the intensity of the laser light varies, the intensity variations are passed through rubidium vapor cell 105; if the frequency of the laser light varies, that affects the resonance of the rubidium atoms, and that in turn affects how much laser light passes through rubidium vapor cell 105.
At 113 is shown a prior-art arrangement for compensating for noise in the laser light. There is added to the arrangement shown at 101 a beam splitter 117, a photo detector 115, a servo control unit 116, and a liquid crystal 117. The amount of light transmitted by the liquid crystal varies according to an input signal from servo control unit 116. Beam splitter 117 provides part of the light from laser light source 103 to photodetector 115 and the remainder to liquid crystal 116 and rubidium vapor cell 105. The output from photodetector 115 goes to servo control unit 116. The output from photodetector 115 varies with the intensity of the light from laser source 105, and the output is used in servo control unit 116 to control liquid crystal 119 such that the amount of light transmitted by liquid crystal 119 remains constant. Liquid crystal 119 thus compensates for variations in the intensity of laser light source 103.
A module which contains beam splitter 117, photodetector 115, servo control unit 116, and liquid crystal 119 is the LPC Laser Power Controller, made by Brockton Electro-Optics Corp., 34 Bellevue, Brockton, Mass. 2302. On Jun. 10, 2003, a description of the module could be downloaded from the Internet at brocktoneo.com/images/lpc—020301/pdf/.
While arrangement 113 does compensate for variations in the intensity of laser light source 103, it can compensate only for intensity noise; it cannot compensate for other noise such as variations in the frequency of the laser light source or noise resulting from photodetector 107; moreover, it is adaptive only with regard to laser light source 103, not with regard to other components of the system. Adaptivity is important in dealing with noise because the properties of an apparatus change as the conditions under which it operates change or as it ages, and the changes in the properties affect both how much noise must be dealt with and the manner in which the apparatus deals with it.
It is thus an object of the invention disclosed herein to provide improved techniques for dealing with the noise in laser light as well as other noise produced by components of the device.